1. Field of the Invention
The present invention relates to an optical disc drive apparatus for recording or reproducing information using an optical disc as a recording medium.
2. Description of the Prior Art
There has heretofore been known such an apparatus as disclosed in Japanese Laid-Open Patent Application No. 58-91536, in which information is recorded along a track with a stroke width of several .mu.m formed on an optical disc. To this end it is necessary to focus a laser beam to a spot by means of an objective lens and apply the spot onto the track accurately. Thus, it is necessary to effect an accurate tracking control. In order to achieve this object it has heretofore been necessary to construct an optical disc drive apparatus with a rough actuator and a fine actuator provided on the rough actuator.
The recording density of an optical disc is about 10 to 100 times higher than that of a magnetic disc. In a comparison of 5-inch one-side discs, a replaceable floppy disc has a memory capacity of about 1MB and a fixed type disc for Winchester drives has a memory capacity of about 10MB, while an optical disc has a memory capacity of about 200MB or even more.
Therefore, an optical disc is so formed as to have a fairly fine track width as compared with that of a magnetic disc, and consequently the accuracy in positioning a laser beam which corresponds to a recording/reproducing head, relative to the track, becomes more strict than that in a magnetic disc apparatus. The track width of a floppy disc is about 500 .mu.m and that of a fixed type disc about several ten .mu.m, while that of an optical disc is at most 2 .mu.m. Positioning a laser beam spot of about 1 .mu.m in diameter accurately onto such a fine track is difficult if positioning means used the for the magnetic disc is used. Besides, since the optical disc, like the floppy disc, is used as a replaceable medium, there occurs a positional error at the time of disc change, with the result that the track does not rotate circularly but rotates eccentrically unlike the rotation of the fixed disc.
Generally, in an optical disc drive apparatus, the position of a laser beam spot relative to a track is corrected using a feedback control, thereby permitting recording or reproduction along the track. To this end, the conventional optical disc drive apparatus is provided with two-stage actuators, namely a rough actuator and a fine actuator.
The rough actuator has a function similar to that of the positioning means for the magnetic disc drive apparatus, in which the laser beam spot is positioned using a mechanical means. The fine actuator is provided on the rough actuator for shifting the laser beam accurately within a specified range and correcting a positional error of the rough actuator.
In such a conventional optical disc drive apparatus, however, the operation for moving the laser beam from one track to another, namely track seek, has been inferior to that of the magnetic disc drive apparatus. In the magnetic disc drive apparatus, what is required is merely moving a recording/reproducing head by means of a positioning mechanism, while in the conventional optical disc drive apparatus the fine actuator must be moved by the rough actuator as mentioned above. The mass of the recording/reproducing head of the magnetic disc drive apparatus is several grams at most, but that of the fine actuator is inevitably several ten grams or more because it carries thereon optical components, e.g. a galvanomirror, for scanning the laser beam. Consequently, the speed of track seek becomes lower in proportion to the increase in conveyance mass. Of more importance is that the precision actuator is a precise and fragile component. Even if the rough actuator will be unable to withstand a large acceleration induced at that time.
There has also been proposed an optical disc drive apparatus without the precision actuator as disclosed, for example, in Japanese Laid-Open Patent Application No. 58-53035, in which an optical head is mounted near the fore end of a rotary actuator. Generally speaking, the positioning accuracy of a rotary actuator is higher than that of a linear actuator. A rotation support mechanism constituted by bearings and a shaft can be sealed hermetically and therefore dust or the like does not adhere thereto, thus permitting a smoother operation than the linear structure. Thus, the rotary actuator can function as both rough and precision actuators.
However, the rotary actuator has such drawbacks that a mechanical resonance is apt to occur as compared with the linear actuator and that therefore it is impossible to increase the response frequency of a tracking servo system and hence it is impossible to increase the number of rotations of the disc. Consequently, the use of an optical disc drive apparatus having such a structure is restricted to the case where the optical disc is rotated at low speed, for example, for digital audio.